Arthropod-borne virus infections remain a major cause of morbidity and mortality worldwide. More than two billion people are at risk of infection with dengue virus (DEN) and 600 million people at risk of infection with yellow fever virus (YF) (20). Globally, an estimated 50-100 million cases of DEN and 200,000 cases of YF are reported each year, which infections result in approximately 20,000 (DEN) and 30,000 (YF) deaths annually (11). There are currently no clinically useable chemotherapeutic options for the treatment of any flavivirus infection, making it essential that new strategies and targets for the treatment of flavivirus infections be identified.
Flaviviruses are small enveloped, single-stranded positive sense RNA viruses with genomes consisting of approximately 11,000 kb RNA with a 5′ type 1 RNA cap (23). The viral genome is translated as a single open reading frame (ORF) encoding a polyprotein precursor that is processed into three structural proteins (capsid, premembrane, and the envelope) and eight nonstructural proteins (NS1, NS2A, NS2B, NS3, NS4A, 2K, NS4B and NS5) by viral and cellular proteases (16). Currently, four viral enzymes are being studied as targets for antiviral drug discovery, including the NS3 helicase and protease enzymes and the NS5 RNA dependent RNA polymerase and capping enzymes (8).
In particular, the capping enzyme has received a good deal of attention as a novel antiviral drug target. The flavivirus capping enzyme has three distinct functions that can be targeted for therapeutic intervention: the N7/2′-O methyltransferase reactions (2, 3, 6, 9) and the recently discovered guanylyltransferase reaction (10, 13, 14). The formation of the 5′ cap structure is critical to the survival of the virus for several reasons, including directing viral polyprotein translation and protecting the 5′ end of the genome from cellular exonucleases. It has also been shown that a fully mature type 1 cap is a mechanism that cells use to discriminate self from non-self RNAs, and interference with the formation of a mature type 1 cap on the flavivirus genome limits viral replication (5, 28).
The flavivirus NS5 N-terminal capping enzyme is highly conserved across the flavivirus genus, and the guanosine triphosphate (GTP) and S-adenosyl methionine (SAM) binding sites, as well as the overall structure of the enzyme, are well conserved (4, 7, 9, 18, 27). The critical nature of the capping enzyme in viral replication and immune evasion, as well as its conservation across the flavivirus genus, position the capping enzyme as an important target for antiviral development efforts. The methyltransferase activity has been the primary capping enzyme target for drug development (15, 21), and ribavirin triphosphate has been observed to bind to and displace GTP from the enzyme (1). However, more effective inhibitors are needed to advance the clinical applications of these efforts.
While arthropod-borne flavivirus infection causes serious morbidity and mortality worldwide, there are currently no effective antiviral chemotherapeutics available for human use. Accordingly, new virus-specific targeting compositions and methods are urgently needed.